With the advent of open plan space interiors (i.e. large interior spaces without floor to ceiling walls) in office buildings, the problem of providing a suitable working environment and sufficient degree of confidentiality within each space has arisen. At least a partial solution to the problem of noise propagation throughout the open space has been in the form of sound masking. The basic concept of sound masking involves the introduction of a speech frequency masking sound, sometimes incorrectly referred to as pink or white noise, into the interior space to increase the background level of sound at certain frequencies. By increasing the background level, ambient noise becomes less noticeable and previously intelligible signals away from their source become less intelligible, thus preventing annoying interference in adjoining spaces. Prior attempts at sound masking have attempted to produce a uniform masking sound throughout the entire uniform space in the belief that it would be less noticeable. This theory, however, was unable to cope with different needs within zones of the interior space. In the past sound masking has been attempted by installing speakers near the sources of noise (such as in U.S. Pat. No. 4,052,564 issued to Propst et al.) or within the enclosed ceiling plenum above such noise sources. The speakers emitting masking sound may be completely self-contained and independently driven or be part of a central system having a single amplifier and a tunable noise generator source and a plurality of output speakers located distant from the central sound source. Such a central masking system is analogous with a central paging system commonly found in office buildings. In the case of a central system, the system is tuned to a predetermined frequency spectrum and sound pressure level generally based on a variation of known preferred noise criteria curves and sound pressure levels recommended in acoustical literature. This type of system is generally a permanent installation and is evaluated in relation to its ability to conform to constant level spectrum criteria. The intent of this type of masking system is to produce a constant sound throughout a large area. These systems have met with some success, but they have severe drawbacks due to their lack of flexibility, as they use predetermined sound levels and spectrum shape to deal with changing and irregular conditions. Irregular conditions result, because of many acoustical variables such as obstruction in the ceiling plenum and varying absorptive characteristics, acoustical panels, and changes in office space layout. Certain frequencies generated by the sound masking device may be absorbed or reflected by these obstructions thereby creating an unevenness of masking sound in the interior space. In order to correct for these variations throughout the space, it would be necessary to install individual masking units having full frequency and amplitude control at nearly every output speaker. In the case of a large interior area, this could become prohibitively expensive and overly complex and is therefore unacceptable. In addition a central masking system is difficult to tune to specification since the frequency and amplitude controls are located at a central location, away from the area which is receiving the masking. Furthermore, due to variations in the electric characteristic in output speakers adjustment at each individual output is necessary to obtain uniformity throughout the space. Fortunately, the present invention overcomes the deficiencies of a central masking system, which suffers from unevenness of output, and the prohibitive cost of using individual masking units with full frequency and amplitude control. By determining zones within the larger interior area which require similar frequency masking characteristics and by providing amplitude control at each output speaker, the difficulties of the prior art can be overcome. Furthermore, by providing frequency and amplitude control right at the output of the masking device, adjustments are easily accomplished to deal with changing acoustical requirements.